The present invention relates to a motor controller and to a control method for a motor, which method controls the motor speed to a desired revolution per minute (r.p.m.) while controlling the input current sinusoidally to improve the power factor and restrain harmonic current distortion; and, the invention also relates to an air-conditioner or refrigerator which employs the motor controller to drive a compressor drive motor so as to condition the air in a room.
The control method described in Japanese Application Patent Laid-Open Publication No. SHO 63-224698 (1988) is a conventional method for operation of a motor controller, which is located in a rectifying circuit that rectifies AC current and converts it to DC power and controls the speed of a motor by use of a combination of a power supply circuit for controlling the DC voltage and a motor driving circuit. This method is used to control the motor speed, when the speed is low, by means of PWM (pulse width modulation) control using the motor driving circuit; and, when the speed is high, the motor speed is controlled by means of PAM (pulse amplitude modulation) control using the DC voltage control of the power supply circuit.
The motor controller described in this J.A.P.L.O.P. No. SHO 63-224698 (1988) will be explained hereunder with reference to FIG. 23 to FIG. 25. In FIG. 23, an inverter control circuit 61 comprises a drive signal generating means 66, which outputs a drive signal for controlling the commutating action of the switching device of an inverter circuit in response to a position detection signal received from a position detection circuit; a PWM signal generation means 67, which generates a PWM signal according to a conduction ratio command, i.e. a speed control signal received from a speed control signal computing means 600; and a driving signal generation means 65, which drives the switching device of the inverter circuit 3 according to the drive signal received from the drive signal generating means 66 and a PWM signal from the PWM signal generating means 67.
A converter control circuit 62 comprises a DC voltage control means 69, which outputs a DC voltage control signal according to the deviation between a DC voltage command, i.e. a speed control signal received from the speed control signal computing means 600, and an actual DC voltage; and an input current sinusoidal control means 68, which controls the current to be inputted to the converter circuit sinusoidally by operating the switching device of a booster chopper circuit in a converter circuit according to the output of the DC voltage control means 69, a source voltage waveform signal and an input current waveform signal.
The operation of the speed control signal computing means 600 will be explained hereunder with reference to FIG. 24. This figure, in which the horizontal axis represents the motor speed (revolutions per minute) and the vertical axis represents the DC voltage command to the converter control circuit 62 and the conduction ratio command to the inverter control circuit 61, shows each operation of the DC voltage command and the conduction ratio command relative to the motor speed (revolutions per minute).
During low-speed operation, the speed control signal computing means 600 outputs a minimum DC voltage command, which is the minimum DC voltage that can control a DC voltage command in the converter circuit, and calculates a conduction ratio command so that the deviation between the speed signal received from the speed computing means 63 and the speed command value received from the outside becomes zero. This is so-called PWM control. By this control, the conduction ratio command increases in direct proportion to the revolutions per minute, as shown in FIG. 24.
During high-speed operation, on the contrary, a conduction ratio command is outputted at 100%, the maximum limit, and a DC voltage command is calculated so that the deviation between the speed signal received from the speed computing means 63 and the speed command value received from the outside becomes zero. This is so-called PAM control. By this control, the DC voltage command increases in direct proportion to the revolutions per minute, as shown in FIG. 24.
There are various methods available for switching to/from the PWM control from/to the PAM control, and one example is a method shown in FIG. 25. Switching from PWM control to PAM control, which is not described here in detail, is accomplished when the conduction ratio command reaches 100% under a condition in which a speed deviation exists and acceleration is needed. On the contrary, switching from PAM control to PWM control is accomplished when the DC voltage command reaches the minimum limit under a condition in which a speed deviation exists and deceleration is needed. By this control, the switching point shifts according to the load condition, and, consequently, stable switching becomes available.
FIG. 24 is based on a precondition that the load torque is constant, and so, if the load torque varies, the speed at which the control is switched also varies. Although the horizontal axis in FIG. 24 represents revolutions per minute, a similar curve can be obtained even if the horizontal axis represents the motor output. That is to say, in such a control method, the control system is switched according to the motor output, i.e. the motor load.
Another control method is described in Japanese Application Patent Laid-Open Publication No. HEI 7-312895 (1995) and No. HEI 8-191589 (1996), wherein the speed of a motor is controlled by PWM control during normal operation, but, after the PWM signal reaches 100% duty, the speed is controlled by the field weakening effect of the commutating phase control during high-speed operation.
According to the method described in the aforementioned J.A.P.L.O.P. No. SHO 63-224698 (1988), control of a motor can be realized in a wide operation range because the control can be switched to/from the PWM control from/to the PAM control according to the load condition. Because the motor can be operated at the minimum DC voltage under PWM control and at the reduced PWM chopper component of an inverter under PAM control, the efficiency of the motor controller can be improved overall. Besides, by employing the unit to drive a compressor of an inverter air-conditioner, it becomes possible to save energy and increase the output, and, consequently, to improve the capacity of the air-conditioner. In particular, the low-temperature heating capacity in case of a low outside temperature improves drastically.
However, under PAM control, since the speed of the motor is increased by increasing the DC voltage, it is impossible to increase the DC voltage above the withstand voltage of the inverter module device. In other words, this is the maximum speed of the motor.
According to the prior method in J.A.P.L.O.P. No. HEI 7-312895 (1995) or No. HEI 8-191589 (1996), on the other hand, the maximum speed of a motor can exceed the limit of the PWM control. However, sufficient consideration is not given to the efficiency of the motor operation under the PWM control.
Besides the above, for an air-conditioner or the like that is equipped with a motor controller employing a combination of PWM and PAM control, it is necessary not only to improve the performance, but also to reduce the cost.
An object of the present invention is to provide a motor controller and a control method for a motor, and also an air-conditioner or the like that employs the controller and/or the control method, wherein the motor speed is controlled by switching to/from the PWM control from/to the PAM control, and in which the motor can be operated always at higher efficiency and in a wider operation range.
Another object of the present invention is to provide a motor controller and a control method for a motor, and also an air-conditioner or the like that employs the controller and/or the control method, wherein the motor speed is controlled by switching to/from the PWM control from/to the PAM control, and in which much higher-speed control is available.
Another object of the present invention is to provide a motor controller and a control method for a motor, and also an air-conditioner or the like that employs the controller and/or the control method, wherein the motor speed is controlled by switching to/from the PWM control from/to the PAM control, and in which the efficiency of the motor under a low-speed operation can be increased.
Another object of the present invention is to provide a motor controller and a control method for a motor, and also an air-conditioner or the like that employs the controller and/or the control method, wherein the motor speed is controlled by switching to/from the PWM control from/to the PAM control, and in which an inverter module becomes available at lower cost.
The above object are achieved by a motor controller or a control method for a motor, said controller comprising a converter circuit that converts AC power to DC, an inverter circuit connected to the output of the converter circuit, a motor connected to the inverter circuit, and a motor control means that controls the speed of the motor. The motor control means comprises a converter control circuit and an inverter control circuit, and it controls the speed of the motor by altering a DC voltage command to the converter control circuit or a conduction ratio command to the inverter control circuit. The motor control means further comprises a commutating phase control means that alters the commutating timing of the coil of the motor, and alters the commutating phase of the coil of the motor according to the speed and load of the motor.
Another characteristic feature of the present invention resides in a motor controller or a control method for a motor, wherein the controller comprises a converter circuit that converts AC power to DC, an inverter circuit connected to the output of the converter circuit, a motor connected to the inverter circuit, and a motor control means that controls the speed of the motor. The motor control means comprises a converter control circuit and an inverter control circuit, and it controls the speed of the motor by altering a DC voltage command to the converter control circuit or a conduction ratio command value to the inverter control circuit. The motor control means further comprises a commutating phase control means that alters the commutating timing of the coil of the motor, said commutating phase control means detecting a value such as input power or DC amperage that relates to the efficiency of the motor controller and alters the commutating phase of the coil of the motor so that the efficiency of the motor controller reaches the maximum limit.
According to the present invention, in using a motor controller or a control method for a motor, wherein the motor speed is controlled by switching to/from the PWM control from/to the PAM control, the efficiency of the motor can be improved by a combination of the PWM/PAM control and independent phase control. That is, in using a motor controller or a control method for a motor, wherein the motor speed is controlled by switching to/from the PWM control from/to the PAM control, the motor can be so controlled as to be able to operate very efficiently in a wider operation range by altering the commutating timing of the coil of the motor using the commutating phase control means and controlling the motor in such a manner that the efficiency becomes the maximum in a steady operation and the speed reaches becomes the maximum limit in a high-speed operation.
The efficiency can be improved particularly by controlling the phase over the entire speed range. Control of the speed in a high-speed range is available either by altering the phase or by altering the DC voltage, and efficient operation can be achieved all the time, provided that which factor to alter is determined in view of which better improves the efficiency. Besides, a similar effect to the above can be obtained by finding an optimum phase for the highest efficiency continuously.
As a result, by applying the invention to an inverter for an air-conditioner, it becomes possible to provide an air-conditioner that has an increased cooling and heating capacity extensively and, at the same time, saves energy. In addition, because an optimum commutating phase can be searched automatically, the above air-conditioner can be provided at lower cost.
Also, according to the present invention, a high-speed operation of a motor becomes available. That is, much higher-speed control becomes available by controlling the phase, without increasing the DC voltage above 330 V. As a result, by applying the invention to an air-conditioner or refrigerator, the capacity in a transient operation, such as at the start-up or at a sudden change in the load, can be increased.
Also according to the present invention, because the design or rated speed of a motor can be decreased, the efficiency in a low-speed operation can be increased. Because the control in a high-speed range leaves some allowance provided that the maximum speed is set constant, the design point of a motor can be lowered. Because of this, the efficiency in a low-speed range can be improved. In addition, by adding the PWM/PAM control to the operation, a much higher-efficiency operation becomes available. As a result, by applying the invention to an air-conditioner or refrigerator, the efficiency in a steady-state operation range improves, thereby resulting in tremendous energy saving and drastically reduced electric energy cost in a year.
Also, according to the present invention, an inverter module becomes available at lower cost. That is, because the maximum DC voltage can be as low as 330 V or so, a standard inverter module becomes applicable, and, hence, an inverter or motor controller can be provided at lower cost. As a result, by applying the invention to an air-conditioner or refrigerator, the cost of each unit can be reduced.